Positive electrode active material and lithium secondary battery comprising the same

ABSTRACT

The present invention relates to a positive electrode active material having improved electrical characteristics by adjusting an aspect ratio gradient of primary particles included in a secondary particle, a positive electrode including the positive electrode active material, and a lithium secondary battery using the positive electrode.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication Nos. 10-2019-0156946, filed on Nov. 29, 2019, and10-2020-0059446, filed on May 19, 2020, the disclosure of which isincorporated herein by reference in their entirety.

BACKGROUND 1. Field of the Invention

The present invention relates to a positive electrode active materialhaving improved electrical characteristics by adjusting an aspect ratiogradient of primary particles included in a secondary particle, apositive electrode including the positive electrode active material, anda lithium secondary battery using the positive electrode.

2. Discussion of Related Art

Batteries store electrical power by using materials facilitating anelectrochemical reaction at a positive electrode and a negativeelectrode. As a representative example of such batteries, there is alithium secondary battery storing electrical energy due to a differencein chemical potential when lithium ions are intercalated/deintercalatedinto/from a positive electrode and a negative electrode.

The lithium secondary battery uses materials enabling reversibleintercalation/deintercalation of lithium ions as positive electrode andnegative electrode active materials, and is manufactured by charging anorganic electrolyte solution or a polymer electrolyte solution betweenthe positive electrode and the negative electrode.

A lithium composite oxide is used as a positive electrode activematerial of the lithium secondary battery, and composite oxides such asLiCoO₂, LiMn₂O₄, LiNiO₂, LiMnO₂, etc. are being studied.

Among the positive electrode active materials, LiCoO₂ is most widelyused due to excellent lifetime characteristics and charge/dischargeefficiency, but it is expensive because of the limited resource ofcobalt, which is used as a raw material, and thus has a disadvantage oflimited price competitiveness.

Lithium manganese oxides such as LiMnO₂ and LiMn₂O₄ have advantages ofexcellent thermal safety and low costs, but also have problems of smallcapacity and poor high-temperature characteristics. In addition, while aLiNiO₂-based positive electrode active material exhibits a batterycharacteristic such as a high discharge capacity, due to cation mixingbetween Li and a transition metal, it is difficult to synthesize theLiNiO₂-based positive electrode active material, thereby causing a bigproblem in rate characteristics.

In addition, depending on the intensification of such cation mixing, alarge amount of Li by-products is generated, and since most of the Liby-products consist of compounds of LiOH and Li₂CO₃, they become a causeof gelation in preparation of a positive electrode paste and gasgeneration according to charge/discharge progression after thepreparation of an electrode. Residual Li₂CO₃ increases the swellingphenomenon of a cell and thus reduces cycles and also leads to theswelling of a battery.

SUMMARY OF THE INVENTION

In the lithium secondary battery market, the growth of lithium secondarybatteries for electric vehicles plays a leading role, and the demand forpositive electrode materials used in lithium secondary batteries is alsoconstantly changing.

For example, conventionally, in terms of securing safety, lithiumsecondary batteries using LFP have been mainly used. However, recently,the use of a nickel-based lithium composite oxide, which has a largerenergy capacity per weight than LFP, is expanding.

In accordance with the trend of such positive electrode materials, thepresent invention is directed to providing to a positive electrodeactive material which has high energy density, an improved lifetime andimproved stability.

Particularly, the present invention is directed to providing a positiveelectrode active material, which includes primary particles enablinglithium intercalation/deintercalation and secondary particles in whichthe primary particles are aggregated, wherein electrical characteristicsare improved by adjusting an aspect ratio gradient or sphericitygradient of the primary particles included in the secondary particle.

In addition, the present invention is directed to providing a positiveelectrode including the positive electrode active material definedherein.

Furthermore, the present invention is directed to providing a lithiumsecondary battery, which uses the positive electrode defined herein.

The objects of the present invention are not limited to theabove-mentioned objects (e.g., for electric cars), and other objects andadvantages of the present invention which are not mentioned can beunderstood by the following description and more clearly understood byexemplary embodiments of the present invention. In addition, it will bereadily apparent that the objects and advantages of the presentinvention may be realized by the means indicated by the claims and acombination thereof.

One aspect of the present invention provides a positive electrode activematerial, which includes primary particles enabling lithiumintercalation/deintercalation and secondary particles formed byaggregating the primary particles.

Here, the primary particles included in the secondary particle mayexhibit an aspect ratio gradient increasing from the core to the surfaceregion of the secondary particle. That is, the average aspect ratio ofthe primary particles may have a gradient increasing from the core tothe surface region of the secondary particle.

Here, by adjusting a gradient range of the average aspect ratio of theprimary particles present in the core of the secondary particle and theaverage aspect ratio of the primary particles present in the surfaceregion of the secondary particle, a decrease in lifetime characteristicsof the positive electrode active material may be minimized, and otherelectrochemical characteristics (e.g., charge capacity, efficiency,output, etc.) may also be improved due to doping of metal elements suchas niobium (Nb) in the positive electrode active material.

Particularly, according to the present invention, there may be atendency for a difference between the average aspect ratio of allprimary particles in the surface region of the secondary particle andthe average aspect ratio of all primary particles in the core of thesecondary particle to be reduced by increasing a mixing ratio of a metaldopant relative to a precursor to increase the content of the metaldopant in the primary particle or specifying a calcination (heattreatment) condition for a mixture of the precursor and the metal dopantin a process of preparing the positive electrode active material.

Therefore, since the primary particles and the secondary particles,which are included in the positive electrode active material, maysatisfy the following conditions, and thus a particle density in thepositive electrode active material may be improved. In addition, theelectrochemical characteristics of the positive electrode activematerial may be improved.

In the positive electrode active material, when a distance from the coreto the surface region of the secondary particle is R, a region where thedistance from the core of the secondary particle is 4/5R to R is a firstregion (R₁), and a region where the distance from the core of thesecondary particle is 0 to 2/5R is a second region (R₂), a ratio (W₁/W₂)of the average aspect ratio (W₁) of all primary particles in the firstregion and the average aspect ratio (W₂) of all primary particles in thesecond region is preferably less than 2.217.

In addition, the ratio (W₁/W₂) of the average aspect ratio (W₁) and theaverage aspect ratio (W₂) may be more than 1.280.

When the primary particles included in the secondary particle show anaspect ratio gradient increasing from the core region to the surfaceregion of the secondary particle such that the average aspect ratio (W₁)of all primary particles in the first region and the average aspectratio (W₂) of all primary particles in the second region have valueswithin the above-mentioned range, a partial strain caused by asymmetricvolume expansion during the charging/discharging of a lithium secondarybattery manufactured using the positive electrode active material can beeffectively reduced, thereby improving electrochemical characteristics.

Another aspect of the present invention provides a positive electrodeincluding the positive electrode active material defined herein.

Still another aspect of the present invention provides a lithiumsecondary battery using the positive electrode defined herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent to those of ordinary skill in theart by describing in detail exemplary embodiments thereof with referenceto the accompanying drawings, in which:

FIG. 1 is a cross-sectional TEM image of a positive electrode activematerial according to Example 1 of the present invention, and FIGS. 2 to5 are partially enlarged images of a region shown in FIG. 1;

FIG. 6 is a cross-sectional TEM image showing a result of measuring theaspect ratio of primary particles in the surface region (first region)of the positive electrode active material according to Example 1 of thepresent invention, and

FIG. 7 is a cross-sectional TEM image showing a result of measuring theaspect ratio of primary particles in the core (second region) of thepositive electrode active material according to Example 1 of the presentinvention;

FIG. 8 is a cross-sectional TEM image of a positive electrode activematerial according to Comparative Example 1, and

FIGS. 9 to 12 are partially enlarged images of the region shown in FIG.8;

FIG. 13 is a cross-sectional TEM image showing the result of measuringthe aspect ratio of primary particles in the surface region (firstregion) of the positive electrode active material according toComparative Example 1, and

FIG. 14 is a cross-sectional TEM image showing the result of measuringthe aspect ratio of primary particles in the core (second region) of thepositive electrode active material according to Comparative Example 1;

FIG. 15 is a cross-sectional TEM image of a positive electrode activematerial according to Comparative Example 2, and

FIGS. 16 to 19 are partially enlarged images of the region shown in FIG.15; and

FIG. 20 is a cross-sectional TEM image showing the result of measuringthe aspect ratio of primary particles in the surface region (firstregion) of the positive electrode active material according toComparative Example 2, and

FIG. 21 shows a cross-sectional TEM image showing the result ofmeasuring the aspect ratio of primary particles in the core (secondregion) of the positive electrode active material according toComparative Example 2.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

To facilitate a better understanding of the present invention, specificterms are defined in the present invention for convenience. Unlessparticularly defined otherwise, scientific and technical terms usedherein will have meanings generally understood by those of ordinaryskill in the art. In addition, it should be understood that, unlessparticularly indicated in the context, the singular forms include pluralforms thereof, and the plural terms also include singular forms thereof.

Hereinafter, a positive electrode active material and a lithiumsecondary battery using a positive electrode including the positiveelectrode active material according to the present invention will bedescribed in further detail.

Positive Electrode Active Material

According to one aspect of the present invention, a positive electrodeactive material including primary particles enabling lithiumintercalation//deintercalation and secondary particles in which theprimary particles are aggregated is provided.

Here, the primary particle means a grain or crystallite, and thesecondary particle means an aggregate formed by aggregating a pluralityof primary particles. There may be a void and/or grain boundary betweenthe primary particles constituting the secondary particles.

In addition, the shape of the primary particle is not particularlylimited, but preferably, the primary particle may have a shapesatisfying an aspect ratio gradient range to be described below.

Generally, in the case of a positive electrode active materialrepresented by the following Formula 1, as the primary particles have anaspect ratio gradient increasing from the core to the surface region ofthe secondary particle, a rod shape may be developed toward the surfaceregion of the secondary particle. Accordingly, primary particles havinga shape relatively close to a spherical shape are mainly present in thecore of the secondary particle, whereas primary particles having a rodshape (an elongated shape extending to one direction) may be present inthe surface region of the secondary particle.

Li_(w)Ni_(1(x+y+z+z′))Co_(X)M1_(y)M2_(z)Nb_(z′)O₂  [Formula 1]

(Here, M1 is at least one selected from Mn and Al,

M2 is at least one selected from P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce,Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu,

M1 and M2 are different elements, and

0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20 and 0≤z′≤0.20) However,according to the present invention, by adjusting the range of an aspectratio gradient of the primary particles in the positive electrode activematerial, the primary particles present in the surface region of thesecondary particle may also have a shape similar to the primaryparticles present in the core of the secondary particle, that is, ashape relatively close to a spherical shape, rather than a relativelyrod shape.

More specifically, according to the present invention, the averageaspect ratio of the primary particles has a gradient increasing from thecore to the surface region of the secondary particle, and with theincrease in content of a metal dopant in the primary particle, there maybe a tendency for decreasing a difference between the average aspectratio of all primary particles in the surface region of the secondaryparticle and the average aspect ratio of all primary particles in thecore of the secondary particle.

Here, as the primary particles may have an average particle of 0.1 to 20nm, preferably, 0.2 to 15 nm, and more preferably, 0.3 to 10 nm, theoptimal density of a positive electrode manufactured using positiveelectrode active materials according to various embodiments of thepresent invention may be realized. In addition, the average diameter ofthe secondary particle may vary according to the average diameter andparticle number of the aggregated primary particles, but may be presentin the range of 0.1 to 25 m, preferably, 2 to 20 m, and more preferably,3 to 15 m.

In one embodiment, the primary particles included in the secondaryparticle exhibit an aspect ratio gradient increasing from the core tothe surface region of the secondary particle.

The term “aspect ratio” used herein is a length/width ratio of the majoraxis (length; a axis) and the minor axis (width; c axis) of the primaryparticle, and when the major axis indicates the direction of arelatively long region of the primary particle, the minor axis indicatesthe length of a relatively short region of the primary particle. Here,the minor axis may indicate the direction perpendicular to the majoraxis.

Accordingly, the overall shape of the primary particle may be determinedby the major axis (a axis) and the length of the minor axis (c axis).For example, when the aspect ratio, which is a ratio of the major axisand the minor axis, of the primary particle is more than 5, the shape ofthe primary particle may be close to a relatively rod shape, rather thana spherical shape. Meanwhile, as the aspect ratio of the primaryparticle is close to 1, the shape of the primary particle will be closeto a spherical shape.

That is, the aspect ratio of the primary particle may be used as anindicator of the sphericity of the primary particle, and it will beunderstood that as the aspect ratio of the primary particle is close to1, the sphericity of the primary particle is greater.

In addition, when the primary particle present in the core of thesecondary particle has a shape close to a spherical shape, and the sizeof an aspect ratio gradient of the primary particles increasing from thecore of the secondary particle to the surface region of the secondaryparticle is small, it will be understood that the size of the sphericitygradient of the primary particles decreasing from the core of thesecondary particle to the surface region of the secondary particle issmall.

According to one embodiment of the present invention, the primaryparticle has an aspect ratio gradient increasing from the core of thesecondary particle to the surface region of the secondary particle, andhere, the size of the aspect ratio gradient satisfies a specificnumerical range to be described. In addition, since the primaryparticles having the above-described aspect ratio gradient pattern arepresent radially from the core of the secondary particle, a straincaused by the volume expansion of the primary particle duringcharging/discharging can be effectively relieved. Therefore, it ispossible to improve the lifetime and stability of a lithium secondarybattery using the positive electrode active material.

In the embodiment, the primary particle may exhibit an aspect ratiogradient continuously increasing from the core of the secondary particleto the surface region of the secondary particle, but the presentinvention is not necessarily limited thereto.

That is, the aspect ratio of the primary particles increases from thecore of the secondary particle to the surface region of the secondaryparticle, and the aspect ratio gradient of the primary particles maycontinuously or discontinuously increase depending on a method ofdefining the gradient.

Here, the primary particles and secondary particles included in thepositive electrode active material may minimize a decrease in lifetimecharacteristics of the positive electrode active material due to dopingof metal elements such as niobium (Nb) in the positive electrode activematerial and improve other electrochemical characteristics (e.g., chargecapacity, efficiency, power, etc.) by at least adjusting a gradientrange of the average aspect ratio of the primary particles present inthe core of the secondary particle and the average aspect ratio of theprimary particles present in the surface region of the secondaryparticle.

Particularly, according to the present invention, the above-describedaspect ratio gradient of the primary particles may be realized byincreasing a mixing ratio of the metal dopant relative to a precursor toincrease the content of the metal dopant in the primary particle orspecifying a calcination (heat treatment) condition for the mixture ofthe precursor of the metal dopant in a process of preparing the positiveelectrode active material.

Specifically, the average aspect ratio of the primary particles includedin the positive electrode active material has a gradient increasing fromthe core of the secondary particle to the surface region of thesecondary particle, and as the metal dopant in the primary particleincreases, a difference between the average aspect ratio of all primaryparticles in the surface region of the secondary particle and theaverage aspect ratio of all primary particles in the core of thesecondary particle is preferably reduced.

That is, it is preferable that the difference between the average aspectratio of the primary particles in the core of the secondary particle andthe average aspect ratio of the primary particles in the surface regionof the secondary particle is not excessively large. Meanwhile, as thecontent of the metal dopant in the primary particle increases in theprocess of preparing the positive electrode active material, when themixing ratio of the metal dopant relative to the precursor isexcessively increased or the calcination (heat treatment) condition forthe mixture of the precursor and the metal dopant is inappropriate(e.g., when a heating rate is excessively high, or an heat treatmenttime is excessively long), the difference between the average aspectratio of the primary particles in the core of the secondary particle andthe average aspect ratio of the primary particles in the surface regionof the secondary particle may be excessively small. In this case, thereis a concern that the reversible efficiency and lifetime characteristicsof the positive electrode active material are degraded.

Here, the primary particles and the secondary particles included in thepositive electrode active material may improve the particle density inthe positive electrode active material by satisfying at least theabove-described conditions. Accordingly, the electrochemicalcharacteristics of the positive electrode active material may beimproved.

In the positive electrode active material, a distance from the core tothe surface region of the secondary particle is R, a region where thedistance from the core of the secondary particle is 4/5R to R is definedas a first region (R₁), and a region where the distance from the core ofthe secondary particle is 0 to 2/5R is defined as a second region (R₂).

For example, when the average particle size of the secondary particlesis 10 m, the first region may be a region having a distance from theoutermost surface of the secondary particle of 0 to 4 μm, and the secondregion may be a region having a distance from the core of the secondaryparticle of 0 to 2 m.

Here, the average aspect ratio (W₁) of all primary particles in thefirst region is preferably less than 4.083, and more preferably 4.0 orless.

When the average aspect ratio (W₁) of all primary particles in the firstregion exceeds the numerical range defined above, the aspect ratio ofthe primary particles present in the surface region of the secondaryparticle compared to that of the core of the secondary particle isexcessively high, and thus the shape of the primary particle present inthe surface region of the secondary particle will be relatively close toa rod shape.

In addition, in order for the aspect ratio of the primary particlespresent in the surface region of the secondary particle to exceed theabove-defined numerical range, the size of the aspect ratio gradient ofthe primary particles increasing from the core of the secondary particleto the surface region of the secondary particle needs to be excessivelylarge.

In this case, it may be difficult to improve the density of the primaryparticles in the positive electrode active material and effectivelyreduce a partial strain caused by the asymmetric volume expansion of thepositive electrode active material during the charging/discharging of alithium secondary battery using the positive electrode active material.That is, due to the excessively increasing difference in aspect ratio ofthe primary particles present in the core of the secondary particle andthe surface region of the secondary particle, it is difficult tosuppress a strain due to volume expansion of the primary particle duringcharging/discharging, thereby increasing the possibility of the positiveelectrode active material cracking. This may act as a cause ofdeteriorating the electrochemical characteristics and/or stability ofthe positive electrode active material.

In addition, as the density of the primary particles in the positiveelectrode active material decreases, the specific surface area of thepositive electrode active material may increase. When the specificsurface area of the positive electrode active material increases, thereis a concern that the lifetime and/or stability of a lithium secondarybattery may be reduced because the possibility of a side reactionbetween the positive electrode active material and an electrolyte in thelithium secondary battery increases.

Meanwhile, the average aspect ratio (W₂) of all primary particles in thesecond region is preferably less than 1.842, and more preferably 1.8 ormore.

When the average aspect ratio (W₂) of all primary particles in thesecond region exceeds the above-defined numerical range, the aspectratio of the primary particles present in the core of the secondaryparticle is excessively high, which may act as the cause of reducing thedensity of the primary particles in the positive electrode activematerial.

In addition, the ratio (W₁/W₂) of the average aspect ratio (W₁) of allprimary particles in the first region and the average aspect ratio (W₂)of all primary particles in the second region is preferably less than2.217. As described above, the positive electrode active materialaccording to the present invention may have improved electricalcharacteristics by reducing the degree of reducing the sphericity of theprimary particle from the core of the secondary particle to the surfaceregion of the secondary particle by adjusting an aspect ratio gradientof the primary particles included in the secondary particle.

That is, as the ratio (W₁/W₂) of the average aspect ratio (W₁) and theaverage aspect ratio (W₂) is allowed to be preferably less than 2.217,and more preferably 2.2 or less, the size of an aspect ratio gradient ofthe primary particles increasing from the core of the secondary particleto the surface region of the secondary particle may be reduced, and thusthe size of a sphericity gradient of the primary particles decreasingfrom the core of the secondary particle to the surface region of thesecondary particle may be reduced.

Likewise, when the ratio (W₁/W₂) of the average aspect ratio (W₁) andthe average aspect ratio (W₂) is more than 2.217, the difference inaspect ratio between the primary particles present in the core of thesecondary particle and the surface region of the secondary particle maybe excessively large, and during charging/discharging, the possibilityof cracking caused by the volume expansion of the primary particles mayincrease.

In addition, the ratio (W₁/W₂) of the average aspect ratio (W₁) and theaverage aspect ratio (W₂) may be at least 1.280, which may mean that atleast the average aspect ratio of the primary particles exhibits agradient increasing from the core of the secondary particle to thesurface region of the secondary particle. As such, since the primaryparticles in the secondary particle exhibit an aspect ratio gradientincreasing from the core of the secondary particle to the surface regionof the secondary particle, and the size of the aspect ratio gradientrepresented by the ratio (W₁/W₂) of the average aspect ratio (W₁) of allprimary particles in the first region and the average aspect ratio (W₂)of all primary particles in the second region is in the range of morethan 1.280 and less than 2.217, the electrical characteristics of thepositive electrode active material present in various trade-offrelationships may be optimized.

Meanwhile, among primary particles in the first region, the proportionof the primary particles having a smaller aspect ratio than the averageaspect ratio (W₁) may be 45% or more. That is, the above-definedproportion of the primary particles is for preventing the average aspectratio (W₁) of all primary particles in the first region from having avalue of 4.083 or more by adjusting the proportion of the primaryparticles having a higher aspect ratio than the average aspect ratio(W₁) among the primary particles in the first region.

In addition, a ratio (W₃/W₂) of the average aspect ratio (W₃) of primaryparticles having a smaller aspect ratio than the average aspect ratio(W₁) among the primary particles in the first region and the averageaspect ratio (W₂) may be more than 0.984 and less than 1.465. That is,in the positive electrode active material having an aspect ratiogradient or sphericity gradient as intended by the present invention,the increment in the aspect ratio of the primary particles present inthe first region is less than that of the aspect ratio of the primaryparticles present in the second region, which indicates that the primaryparticles present in the first region may have a shape relatively closeto a spherical shape.

Meanwhile, a ratio (W₄/W₂) of the average aspect ratio (W₄) of theprimary particles having a higher aspect ratio than the average aspectratio (W₁) among the primary particles in the first region and theaverage aspect ratio (W₂) may be more than 1.793 and less than 3.076.This means that, in the positive electrode active material having anaspect ratio gradient or sphericity gradient as intended by the presentinvention, even primary particles with a larger increment in aspectratio of the primary particles present in the second region among theprimary particles present in the first region do not have a perfect rodshape with an extremely large aspect ratio.

The primary particles satisfying the above-described various conditionsmay be defined as a lithium composite oxide represented by Formula 1below.

Li_(w)Ni_(1(x+y+z+z′))Co_(x)M1_(y)M2_(z)Nb_(z′)O₂  [Formula 1]

(Here, M1 is at least one selected from Mn and Al,

M2 is at least one selected from P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce,Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu,

M1 and M2 are different elements, and

0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, and 0≤z′≤0.20.)

Here, a difference between the average aspect ratio of all primaryparticles in the surface region of the secondary particle and theaverage aspect ratio of all primary particles in the core of thesecondary particle, and particularly, the ratio (W₁/W₂) of the averageaspect ratio (W₁) and the average aspect ratio (W₂) may decrease, as thez′ value increases. In addition, the ratio (W₁/W₂) of the average aspectratio (W₁) and the average aspect ratio (W₂) may be reduced byspecifying a calcination (heat treatment) condition for a mixture of aprecursor of the lithium composite oxide represented by Formula 1 and ametal dopant (Nb-containing raw material).

In addition, M2 and/or niobium (Nb) present in the surface region of thesecondary particle may exhibit a concentration gradient decreasingtoward the core of the secondary particle. That is, the direction of theconcentration gradient of M2 and/or niobium (Nb) may be a direction fromthe surface region of the secondary particle to the core of thesecondary particle.

Particularly, a lithium ion diffusion path in the primary particle maybe formed in the same direction as the direction of the concentrationgradient of M2 and/or niobium (Nb), that is, a direction from thesurface region of the secondary particle to the core of the secondaryparticle. The lithium ion diffusion path is formed in the same directionas that of the concentration gradient of M2 and/or niobium (Nb) (or adirection from the core of the secondary particle to the surface regionof the secondary particle), or to form an angle within ±400 with respectto a virtual straight light connecting the core of the secondaryparticle with the surface region of the secondary particle.

Like this, as the lithium ion diffusion path in the primary particle isformed in a direction from the core of the secondary particle to thesurface region of the secondary particle, the diffusion property oflithium ions in the positive electrode active material may be improved,and it may contribute to the improvement in electrical characteristicsof the positive electrode active material.

In addition, according to another embodiment, the positive electrodeactive material according to the present invention may include a coatinglayer covering at least a part of the surface(s) of the primary particle(e.g., the interface between the primary particles) and/or the secondaryparticle formed by aggregating the primary particles.

For example, the coating layer may be present to cover at least a partof an exposed surface of the primary particle. Particularly, the coatinglayer may be present to cover at least a part of the exposed surface ofthe primary particle present on the outermost surface of the secondaryparticle.

Accordingly, the coating layer may be present as a layer continuously ordiscontinuously coating the surface(s) of the primary particle and/orthe secondary particle formed by aggregating the primary particles. Whenthe coating layer is present discontinuously, it may have an islandshape.

The coating layer present as above may contribute to the improvement ofphysical and electrochemical characteristics of the positive electrodeactive material.

In addition, the coating layer may be present as a solid solution whichdoes not form a boundary between the primary particle and/or thesecondary particles formed by aggregating the primary particles.

The coating layer may include at least one oxide represented by Formula2 below. That is, the coating layer may be defined as a region in whichan oxide represented by Formula 2 below is present.

Li_(a)M3_(b)O_(c)  [Formula 2]

(Here, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti,Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta,Sn, Hf, Ce, Gd and Nd, and

0≤a≤10, 0≤b≤8, and 2≤c≤13.)

In addition, the coating layer may have a form in which heterogeneousoxides are simultaneously present in one layer, or heterogeneous oxidesrepresented by Formula 2 as described above are present in separatelayers, respectively.

The oxide represented by Formula 2 may be physically and/or chemicallybonded with the primary particle represented by Formula 1. In addition,the oxide may form a solid solution with the primary particlerepresented by Formula 1.

The positive electrode active material according to the embodiment mayinclude a coating layer covering at least a part of the surface(s) ofthe primary particle (e.g., the interface between the primary particles)and/or the secondary particle formed by aggregating the primaryparticles, thereby increasing structural stability. In addition, whenthe positive electrode active material is used in a lithium secondarybattery, the high temperature storage stability and lifetimecharacteristics of the positive electrode active material may beimproved. In addition, the oxide may reduce residual lithium in thepositive electrode active material and act as a lithium ion migrationpathway, and thus may affect the improvement in efficiencycharacteristics of the lithium secondary battery.

In addition, in some cases, the oxide may be present not only in atleast a part of the interface between the primary particles and thesurface of the secondary particle, but also in an inner void formed inthe secondary particle.

The oxide may be an oxide in which lithium and an element represented byA are combined or an oxide of A, and the oxide may be, for example,LiaWbOc, LiaZrbOc, LiaTibOc, LiaNibOc, LiaBbOc, LiaCObOc, LiaAlOc,CObOc, AlbOc, WbOc, ZrbOc, TibOc or BbOc, but the above examples aremerely described for convenience to help in understanding, and the oxidedefined in the present invention is not limited to the above-describedexamples.

In another embodiment, the oxide may be or further include an oxide inwhich lithium and at least two types of elements represented by A arecombined. The oxide in which lithium and at least two types of elementsrepresented by A are combined may be, for example,Li_(a)(W/Ti)_(b)O_(c), Li_(a)(W/Zr)_(b)O_(c), Li_(a)(W/Ti/Zr)_(b)O_(c),or Li_(a)(W/Ti/B)_(b)O_(c), but the present invention is not necessarilylimited thereto.

Here, the oxide may exhibit a concentration gradient decreasing from thesurface region of the secondary particle to the core of the secondaryparticle. Accordingly, the concentration of the oxide may decrease fromthe outermost surface of the secondary particle to the core of thesecondary particle.

As described above, as the oxide exhibits a concentration gradientdecreasing from the surface region of the secondary particle to the coreof the secondary particle, residual lithium present on the surface ofthe positive electrode active material may be effectively reduced,thereby preventing a side reaction caused by unreacted residual lithiumin advance. In addition, a decrease in crystallinity in the innersurface region of the positive electrode active material caused by theoxide may be prevented. In addition, the breakdown of the entirestructure of the positive electrode active material due to the oxide inan electrochemical reaction may be prevented.

In addition, the coating layer may include a first oxide layer includingat least one oxide represented by Formula 2, and a second oxide layerwhich includes at least one oxide represented by Formula 2 and adifferent oxide from the oxide included in the first oxide layer.

For example, the first oxide layer may be present to cover at least apart of the exposed surface of the primary particle present on theoutermost surface of the secondary particle, and the second oxide layermay be present to cover at least a part of the exposed surface of theprimary particle not covered by the first oxide layer and the surface ofthe first oxide layer.

Lithium Secondary Battery

Another aspect of the present invention may provide a positive electrodeincluding a positive electrode current collector and a positiveelectrode active material layer formed on the positive electrode currentcollector. Here, the positive electrode active material layer mayinclude, as a positive electrode active material, any one of the lithiumcomposite oxides according to various embodiments of the presentinvention described above. Therefore, since the positive electrodeactive material is the same as described above, detailed descriptionthereof will be omitted for convenience, and hereinafter, othercomponents that have not been described will be described.

The positive electrode current collector is not particularly limited aslong as it does not cause a chemical change in a battery and hasconductivity, and for example, stainless steel, aluminum, nickel,titanium, calcined carbon, or aluminum or stainless steel whose surfaceis treated with carbon, nickel, titanium or silver may be used. Inaddition, the positive electrode current collector may conventionallyhave a thickness of 3 to 500 m, and fine irregularities may be formed onthe surface of the current collector, thereby increasing the adhesivestrength of a positive electrode active material. For example, thepositive electrode current collector may be used in various forms suchas a film, a sheet, a foil, a net, a porous body, foam, a non-wovenfabric, etc.

The positive electrode active material layer may be prepared by coatingthe positive electrode current collector with a positive electrodeslurry composition including the positive electrode active material, aconductive material, and a binder included selectively as needed.

Here, the positive electrode active material is included at 80 to 99 wt%, and specifically, 85 to 98.5 wt % with respect to the total weight ofthe positive electrode active material layer. When the positiveelectrode active material is included in the above content range,excellent capacity characteristics may be exhibited, but the presentinvention is not limited thereto.

The conductive material is used to impart conductivity to an electrode,and is not particularly limited as long as it has electron conductivitywithout causing a chemical change in a battery. A specific example ofthe conductive material may be graphite such as natural graphite orartificial graphite; a carbon-based material such as carbon black,acetylene black, Ketjen black, channel black, furnace black, lamp black,thermal black or a carbon fiber; a metal powder or metal fiberconsisting of copper, nickel, aluminum, or silver; a conductive whiskerconsisting of zinc oxide or potassium titanate; a conductive metal oxidesuch as titanium oxide; or a conductive polymer such as a polyphenylenederivative, and one or a mixture of two or more thereof may be used. Theconductive material may be generally contained at 0.1 to 15 wt % withrespect to the total weight of the positive electrode active materiallayer.

The binder serves to improve attachment between particles of thepositive electrode active material and the adhesive strength between thepositive electrode active material and a current collector. A specificexample of the binder may be polyvinylidene fluoride (PVDF), avinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC),starch, hydroxypropylcellulose, regenerated cellulose,polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,an ethylene-propylene-diene polymer (EPDM), a sulfonated EPDM, styrenebutadiene rubber (SBR), fluorine rubber, or various copolymers thereof,and one or a mixture of two or more thereof may be used. The binder maybe included at 0.1 to 15 wt % with respect to the total weight of thepositive electrode active material layer.

The positive electrode may be manufactured according to a conventionalmethod of manufacturing a positive electrode, except that theabove-described positive electrode active material is used.Specifically, the positive electrode may be manufactured by applying apositive electrode slurry composition prepared by dissolving ordispersing the positive electrode active material, and selectively, abinder and a conductive material in a solvent on the positive electrodecurrent collector, and drying and rolling the resulting product.

The solvent may be a solvent generally used in the art, and may bedimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP),acetone or water, and one or a mixture of two or more thereof may beused. In consideration of the coating thickness and production yield ofa slurry, the solvent is used at a sufficient amount for dissolving ordispersing the positive electrode active material, the conductivematerial and the binder and then imparting a viscosity for exhibitingexcellent thickness uniformity when the slurry is applied to prepare apositive electrode.

In addition, in another exemplary embodiment, the positive electrode maybe prepared by casting the positive electrode slurry composition on aseparate support, and laminating a film obtained by delamination fromthe support on the positive electrode current collector.

Still another aspect of the present invention provides anelectrochemical device including the above-described positive electrode.The electrochemical device may be, specifically, a battery, a capacitor,and more specifically, a lithium secondary battery.

The lithium secondary battery may specifically include a positiveelectrode, a negative electrode disposed opposite to the positiveelectrode, and a separator and an electrolyte, which are interposedbetween the positive electrode and the negative electrode. Here, sincethe positive electrode is the same as described above, for convenience,detailed description for the positive electrode will be omitted, andother components which have not been described below will be describedin detail.

The lithium secondary battery may further include a battery caseaccommodating an electrode assembly of the positive electrode, thenegative electrode and the separator, and selectively, a sealing memberfor sealing the battery case.

The negative electrode may include a negative electrode currentcollector and a negative electrode active material layer disposed on thenegative electrode current collector.

The negative electrode current collector is not particularly limited aslong as it has high conductivity without causing a chemical change in abattery, and may be, for example, copper, stainless steel, aluminum,nickel, titanium, calcined carbon, or copper or stainless steel whosesurface is treated with carbon, nickel, titanium or silver, or analuminum-cadmium alloy. In addition, the negative electrode currentcollector may generally have a thickness of 3 to 500 μm, and like thepositive electrode current collector, fine irregularities may be formedon the current collector surface, thereby enhancing the binding strengthof the negative electrode active material. For example, the negativeelectrode current collector may be used in various forms such as a film,a sheet, a foil, a net, a porous body, foam, a non-woven fabric, etc.

The negative electrode active material layer may be formed by coatingthe negative electrode current collector with a negative electrodeslurry composition including the negative electrode active material, aconductive material and a binder selectively included as needed.

As the negative electrode active material, a compound enabling thereversible intercalation and deintercalation of lithium may be used. Aspecific example of the negative electrode active material may be acarbonaceous material such as artificial graphite, natural graphite,graphitized carbon fiber or amorphous carbon; a metallic compoundcapable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In,Mg, Ga, Cd, a Si alloy, a Sn alloy or an Al alloy; a metal oxide capableof doping and dedoping lithium such as SiO_(β) (0<β<2), SnO₂, vanadiumoxide, or lithium vanadium oxide; or a composite including the metalliccompound and the carbonaceous material such as a Si—C composite or aSn—C composite, and any one or a mixture of two or more thereof may beused. In addition, as the negative electrode active material, a metallithium thin film may be used. In addition, as a carbon material, bothlow-crystalline carbon and high-crystalline carbon may be used.Representative examples of the low-crystalline carbon include softcarbon and hard carbon, and representative examples of thehigh-crystalline carbon include amorphous, sheet-type, flake-type,spherical or fiber-type natural or artificial graphite, Kish graphite,pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbonmicrobeads, mesophase pitches, and high-temperature calcined carbon suchas petroleum or coal tar pitch derived cokes.

The negative electrode active material may be included at 80 to 99 wt %with respect to the total weight of the negative electrode activematerial layer.

The binder is a component aiding bonding between a conductive material,an active material and a current collector, and may be generally addedat 0.1 to 10 wt % with respect to the total weight of the negativeelectrode active material layer. Examples of the binder may includepolyvinylidene fluoride (PVDF), polyvinyl alcohol,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM),sulfonated-EPDM, styrene butadiene rubber, nitrile-butadiene rubber,fluorine rubber, and various copolymers thereof.

The conductive material is a component for further improvingconductivity of the negative electrode active material, and may be addedat 10 wt % or less, and preferably, 5 wt % or less with respect to thetotal weight of the negative electrode active material layer. Theconductive material is not particularly limited as long as it does notcause a chemical change in the battery, and has conductivity, and maybe, for example, graphite such as natural graphite or artificialgraphite; carbon black such as acetylene black, Ketjen black, channelblack, furnace black, lamp black or thermal black; a conductive fibersuch as a carbon fiber or a metal fiber; a metal powder such asfluorinated carbon, aluminum, or nickel powder; a conductive whiskersuch as zinc oxide or potassium titanate; a conductive metal oxide suchas titanium oxide; or a conductive material such as a polyphenylenederivative.

In an exemplary embodiment, the negative electrode active material layermaybe prepared by applying a composition for forming the negativeelectrode active material layer, which is prepared by dissolving ordispersing a negative electrode active material, and selectively abinder and a conductive material in a solvent, on the negative electrodecurrent collector, and drying the resulting product, or may be preparedby casting a composition for forming the negative electrode activematerial layer on a separate support and laminating a film obtained bydelamination from the support on the negative electrode currentcollector.

In addition, in another exemplary embodiment, the negative electrodeactive material layer may be prepared by applying a composition forforming the negative electrode active material layer, which is preparedby dissolving or dispersing a negative electrode active material, andselectively a binder and a conductive material in a solvent, on thenegative electrode current collector, and drying the resulting product,or may be prepared by casting a composition for forming the negativeelectrode active material layer on a separate support, and thenlaminating a film obtained by delamination from the support on thenegative electrode current collector.

Meanwhile, in the lithium secondary battery, a separator is notparticularly limited as long as it is generally used in a lithiumsecondary battery to separate a negative electrode from a positiveelectrode and provide a diffusion path for lithium ions, andparticularly, the separator has a low resistance to ion mobility of anelectrolyte and an excellent electrolyte solution impregnation ability.Specifically, a porous polymer film, for example, a porous polymer filmprepared of a polyolefin-based polymer such as an ethylene homopolymer,a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer and an ethylene/methacrylate copolymer, or astacked structure including two or more layers thereof may be used. Inaddition, a conventional porous non-woven fabric, for example, anon-woven fabric formed of a high melting point glass fiber or apolyethylene terephthalate fiber may be used. In addition, a coatedseparator including a ceramic component or a polymer material may beused to ensure thermal resistance or mechanical strength, and may beselectively used in a single- or multi-layered structure.

In addition, the electrolyte used in the present invention may be anorganic liquid electrolyte, an inorganic liquid electrolyte, a solidpolymer electrolyte, a gel-type polymer electrolyte, a solid inorganicelectrolyte, or a molten-type inorganic electrolyte, which can be usedin production of a lithium secondary battery, but the present inventionis not limited thereto.

Specifically, the electrolyte may include an organic solvent and alithium salt.

The organic solvent is not particularly limited as long as it can serveas a medium enabling the transfer of ions involved in an electrochemicalreaction of a battery. Specifically, the organic solvent may be anester-based solvent such as methyl acetate, ethyl acetate,7-butyrolactone, or F-caprolactone; an ether-based solvent such asdibutyl ether or tetrahydrofuran; a ketone-based solvent such ascyclohexanone; an aromatic hydrocarbon-based solvent such as benzene orfluorobenzene; a carbonate-based solvent such as dimethyl carbonate(DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate(PC); an alcohol-based solvent such as ethyl alcohol or isopropylalcohol; a nitrile-based solvent such as R—CN (R is a linear, branchedor cyclic C2 to C20 hydrocarbon group, and may include a double bondedaromatic ring or an ether bond); an amide-based solvent such asdimethylformamide; a dioxolane-based solvent such as 1,3-dioxolane; or asulfolane-based solvent. Among these, a carbonate-based solvent ispreferably used, and a mixture of a cyclic carbonate (for example,ethylene carbonate or propylene carbonate) having high ion conductivityand high permittivity to increase the charge/discharge performance of abattery and a low-viscosity linear carbonate-based compound (forexample, ethyl methyl carbonate, dimethyl carbonate or diethylcarbonate) is more preferably used. In this case, by using a mixture ofa cyclic carbonate and a chain-type carbonate in a volume ratio of about1:1 to about 1:9, the electrolyte solution may exhibit excellentperformance.

The lithium salt is not particularly limited as long as it is a compoundcapable of providing a lithium ion used in a lithium secondary battery.Specifically, the lithium salt may be LiPF₆, LiClO₄, LiAsF₆, LiBF₄,LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂,LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, LiCl, LiI, or LiB(C₂O₄)₂. The concentrationof the lithium salt is preferably in the range of 0.1 to 2.0M. When theconcentration of the lithium salt is included in the above-mentionedrange, the electrolyte has suitable conductivity and viscosity and thuscan exhibit excellent electrolytic performance. Therefore, lithium ionscan effectively migrate.

To enhance lifespan characteristics of the battery, inhibit a decreasein battery capacity, and enhance discharge capacity of the battery, theelectrolyte may further include one or more types of additives, forexample, a haloalkylene carbonate-based compound such asdifluoroethylene carbonate, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphorictriamide, a nitrobenzene derivative, sulfur, a quinone imine dye,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxy ethanol oraluminum trichloride, in addition to the components of the electrolyte.Here, the additive(s) may be included at 0.1 to 5 wt % with respect tothe total weight of the electrolyte.

Since the lithium secondary battery including the positive electrodeactive material according to the present invention stably exhibitsexcellent discharge capacity, excellent output characteristics andexcellent lifespan characteristics, it is useful in portable devicessuch as a mobile phone, a notebook computer and a digital camera and anelectric automobile field such as a hybrid electric vehicle (HEV).

The outer shape of the lithium secondary battery according to thepresent invention is not particularly limited, but may be a cylindrical,prismatic, pouch or coin type using a can. In addition, the lithiumsecondary battery may be used in a battery cell that is not only used asa power source of a small device, but also preferably used as a unitbattery for a medium-to-large battery module including a plurality ofbattery cells.

According to yet another exemplary embodiment of the present invention,a battery module including the lithium secondary battery as a unit celland/or a battery pack including the same is provided.

The battery module or the battery pack may be used as a power source ofany one or more medium-to-large devices including a power tool; anelectric motor vehicle such as an electric vehicle (EV), a hybridelectric vehicle, and a plug-in hybrid electric vehicle (PHEV); and apower storage system.

Hereinafter, the present invention will be described in further detailwith reference to examples. However, these examples are merely providedto explain the present invention, and it will not be interpreted thatthe scope of the present invention is limited by the examples below.

Preparation Example 1. Preparation of Positive Electrode Active Material(1) Example 1

A spherical Ni_(0.80)Co_(0.10)Mn_(0.10)(OH)₂ hydroxide precursor wassynthesized by a co-precipitation method. Specifically, in a 90 Lreactor, 25 wt % NaOH and 30 wt % NH₄OH were input into an aqueous 1.5Msulfuric acid solution of a transition metal composite in which nickelsulfate, cobalt sulfate and manganese sulfate are mixed in a molar ratioof 80:1:1. The pH in the reactor was maintained at 11.5, the temperatureof the reactor was maintained at 60° C., and an inert gas N₂ was addedto the reactor to prevent the prepared precursor from being oxidized.After the completion of synthesis and stirring, washing and dehydrationwere performed using filter press (F/P) equipment, thereby obtaining aNi_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ hydroxide precursor.

Subsequently, a lithium composite oxide was prepared by adding LiOH andan Nb-containing raw material (Nb₂O₅) to the synthesized precursor andcalcinating the same. Specifically, a lithium composite oxide wasobtained by mixing LiOH and an Nb-containing raw material (Nb₂O₅) with aprecursor, subjected to heat treatment for 15 hours in a calcinationfurnace by raising a temperature 1° C. per minute until 830° C. in an 02atmosphere, and naturally cooled. The Nb-containing raw material (Nb₂O₅)was mixed to be 0.5 mol % in the total composition before calcination.

Subsequently, the obtained lithium composite oxide was subjected to heattreatment in a calcination furnace for 20 hours by raising thetemperature 2° C. per minute until 700° C. in an 02 atmosphere and thennaturally cooled.

(2) Example 2

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that the Nb-containing raw material(Nb₂O₅) was added to be 1.0 mol % in the total composition beforecalcination, and heat treatment was performed in a calcination furnaceby raising a temperature 1° C. per minute until 860° C. in an 02atmosphere.

(3) Example 3

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that a sphericalNi_(0.94)Co_(0.03)Mn_(0.03)(OH)₂ hydroxide precursor was synthesized bya co-precipitation method, and then LiOH and an Nb-containing rawmaterial (Nb₂O₅) were added thereto and calcined, thereby preparing alithium composite oxide, and the Nb-containing raw material (Nb₂O₅) wasadded to be 1.0 mol % in the total composition before calcination, andheat treatment was performed in a calcination furnace by raising atemperature 1° C. per minute until 780° C. in an 02 atmosphere.

(4) Comparative Example 1

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that the Nb-containing raw material(Nb₂O₅) was not mixed before calcination, and heat treatment wasperformed in a calcination furnace by raising a temperature 1° C. perminute until 810° C. in an O₂ atmosphere.

(5) Comparative Example 2

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that the Nb-containing raw material(Nb₂O₅) was mixed to be 2.0 mol % in the total composition beforecalcination, and heat treatment was performed in a calcination furnaceby raising a temperature 1° C. per minute until 860° C. in an O₂atmosphere.

(6) Comparative Example 3

A positive electrode active material was prepared by the same method asdescribed in Example 3, except that the Nb-containing raw material(Nb₂O₅) was mixed to be 0.5 mol % in the total composition beforecalcination, and heat treatment was performed in a calcination furnaceby raising a temperature 1° C. per minute until 760° C. in an O₂atmosphere.

(7) Comparative Example 4

A positive electrode active material was prepared by the same method asdescribed in Example 3, except that the Nb-containing raw material(Nb₂O₅) was mixed to be 2.0 mol % in the total composition beforecalcination, and heat treatment was performed in a calcination furnaceby raising a temperature 1° C. per minute until 820° C. in an 02atmosphere.

(8) Comparative Example 5

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that LiOH and an Nb-containing rawmaterial (Nb₂O₅) were added to a synthesized precursor to be 0.5 mol %in the total composition, and the mixture was subjected to heattreatment for 15 hours in a calcination furnace by raising a temperature5° C. per minute until 830° C. in an 02 atmosphere and then naturallycooled.

(9) Comparative Example 6

A positive electrode active material was prepared by the same method asdescribed in Example 1, except that LiOH and an Nb-containing rawmaterial (Nb₂O₅) were added to a synthesized precursor to be 0.5 mol %in the total composition, and the mixture was subjected to heattreatment for 20 hours in a calcination furnace by raising a temperature1° C. per minute until 830° C. in an 02 atmosphere and then naturallycooled.

Preparation Example 2. Preparation of Lithium Secondary Battery

A positive electrode slurry was prepared by dispersing 94 wt % of thepositive electrode active material prepared according to PreparationExample 1, 3 wt % of carbon black and 3 wt % of a PVDF binder in 30 g ofN-methyl-2 pyrrolidone (NMP). A positive electrode was formed byapplying and drying the positive electrode slurry on an aluminum (Al)thin film, which is a positive electrode current collector, with athickness of 15 m, and performing roll pressing. The positive electrodehad a loading level of 10 mg/cm², and an electrode density of 3.2 g/cm³.

A metal lithium was used as a counter electrode for the positiveelectrode, and an electrolyte solution was prepared by adding 1.15MLiPF₆ to a solvent prepared by mixing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of2:4:4.

A lithium secondary battery (coin cell) was prepared by forming abattery assembly by interposing a separator formed of a porouspolyethylene (PE) film between the positive electrode and the negativeelectrode, and injecting the electrolyte solution thereinto.

Experimental Example 1. TEM Analysis of Positive Electrode ActiveMaterial

Each of the positive electrode active materials (secondary particle)according to Example 1, Comparative Example 1 and Comparative Example 2was subjected to cross-sectioning with a cross-section polisher(accelerating voltage: 5.0 kV, 4-hour milling), and then cross-sectionalTEM images were obtained. Here, a region in which the average particlesize of the positive electrode active material was 10 μm, and on thecross-sectional TEM image, where a distance from the outermost surfaceof the positive electrode active material is 0 to 4 μm was defined as asurface region (first region), and a region where a distance from thecore of the positive electrode active material is 0 to 2 m was definedas a core (second region).

FIG. 1 is a cross-sectional TEM image of the positive electrode activematerial according to Example 1 of the present invention, and FIGS. 2 to5 are partially enlarged images of the region shown in FIG. 1.

FIG. 6 is a cross-sectional TEM image showing a result of measuring theaspect ratio of primary particles in the surface region (first region)of the positive electrode active material according to Example 1 of thepresent invention, and FIG. 7 is a cross-sectional TEM image showing aresult of measuring the aspect ratio of primary particles in the core(second region) of the positive electrode active material according toExample 1 of the present invention.

Referring to FIGS. 1 to 7, it can be confirmed that the positiveelectrode active material according to Example 1 has an aspect ratiogradient increasing from the core of the secondary particle to thesurface region of the secondary particle, and the shape of the primaryparticle present in the first region (first section, second section andfourth section) is relatively close to a spherical form. Particularly,referring to the result of measuring an aspect ratio shown in FIGS. 6and 7, the increment in aspect ratio of the primary particles present inthe first region compared with the primary particle present in thesecond region is not excessively large, indicating that the aspect ratioor sphericity of the primary particle present in the first region islarger than that of Comparative Example 1.

FIG. 8 is a cross-sectional TEM image of a positive electrode activematerial according to Comparative Example 1, and FIGS. 9 to 12 arepartially enlarged images of the region shown in FIG. 8.

FIG. 13 is a cross-sectional TEM image showing the result of measuringthe aspect ratio of primary particles in the surface region (firstregion) of the positive electrode active material according toComparative Example 1, and FIG. 14 is a cross-sectional TEM imageshowing the result of measuring the aspect ratio of primary particles inthe core (second region) of the positive electrode active materialaccording to Comparative Example 1.

Referring to FIGS. 8 to 12, the positive electrode active materialaccording to Comparative Example 1 has an aspect ratio gradientincreasing from the core of the secondary particle to the surface regionof the secondary particle, and particularly, it can be confirmed thatthe shape of the primary particle present in the first region (thefirst, second and fourth sections) is a rod shape. In addition,referring to the results of measuring an aspect ratio shown in FIGS. 13and 14, it can be confirmed that the increment in aspect ratio of theprimary particles present in the first region compared with that presentin the second region is excessively large, and the aspect ratio orsphericity of the primary particle present in the first region is lowerthan that of Example 1.

FIG. 15 is a cross-sectional TEM image of a positive electrode activematerial according to Comparative Example 2, and FIGS. 16 to 19 arepartially enlarged images of the region shown in FIG. 15.

FIG. 20 is a cross-sectional TEM image showing the result of measuringthe aspect ratio of primary particles in the surface region (firstregion) of the positive electrode active material according toComparative Example 2, and FIG. 21 shows a cross-sectional TEM imageshowing the result of measuring the aspect ratio of primary particles inthe core (second region) of the positive electrode active materialaccording to Comparative Example 2.

Referring to FIGS. 15 to 19, it can be confirmed that the positiveelectrode active material according to Comparative Example 2 hardlyexhibits an aspect ratio gradient increasing from the core of thesecondary particle to the surface region of the secondary particle, andthe shape of the primary particle present in the first region (thefirst, second and fourth sections) is almost similar to that of theprimary particle present in the second region (the third section). Theseresults can also be confirmed from the results of measuring an aspectratio shown in FIGS. 20 and 21.

In the following Tables 1 and 2, the results of measuring the averageaspect ratio (W₁) in the first region, the average aspect ratio (W₂) inthe second region, the average aspect ratio (W₃) of first particleshaving a smaller aspect ratio than the average aspect ratio (W₁) amongthe primary particles in the first region, and the average aspect ratio(W₄) of primary particles having a larger aspect ratio than the averageaspect ratio (W₁) among the primary particles in the first region of thepositive electrode active material prepared according to the preparationexample are shown. The average values are average values of the aspectratios of at least 100 primary particles in respective regions.

W₁: the average aspect ratio of all primary particles in a first region(a region where the distance from the outermost surface of a positiveelectrode active material is 0 to 4 μm)

W₂: the average aspect ratio of all primary particles in a second region(a region where the distance from the core of a positive electrodeactive material is 0 to 2 μm)

W₃: the average aspect ratio of primary particles having a smalleraspect ratio than W₁ among primary particles in a first region

TABLE 1 Classification Example 1 Example 2 Example 3 W₁ 2.222 1.8982.775 W₂ 1.478 1.401 1.642 W₁/W₂ 1.504 1.355 1.690 W₃ 1.596 1.400 1.998W₃/W₂ 1.080 0.999 1.217 W₄ 3.043 2.717 3.897 W₄/W₂ 2.059 1.939 2.373 P56.2% 58.9% 57.9%

W₄: the average aspect ratio of primary particles having a larger aspectratio than W₁ among primary particles in a first region

P: the proportion of primary particles having a smaller aspect ratiothan W₁ among primary particles in a first region

TABLE 2 Comparative Comparative Comparative Comparative ComparativeComparative Classification Example 1 Example 2 Example 3 Example 4Example 5 Example 6 W₁ 4.083 1.569 3.855 1.912 1.746 1.618 W₂ 1.8421.226 1.716 1.512 1.410 1.375 W₁/W₂ 2.217 1.280 2.247 1.265 1.238 1.177W₃ 2.699 1.206 2.519 1.471 1.211 1.195 W₃/W₂ 1.465 0.984 1.468 0.9730.859 0.869 W₄ 5.666 2.146 5.415 2.711 2.251 2.103 W₄/W₂ 3.076 1.7503.156 1.793 1.596 1.529 P 44.7% 61.3% 42.9% 64.7% 59.2% 60.5%

Experimental Example 2. Evaluation of Battery Capacity and LifetimeCharacteristics of Lithium Secondary Battery

The lithium secondary battery manufactured according to PreparationExample 2 was subjected to a charging/discharging test using anelectrochemical analyzer (Toyo, Toscat-3100) by applying a voltage rangeof 3.0V to 4.3V and a discharge rate of 0.2 C to 5.0 C at 25° C. tomeasure an initial charging capacity, an initial discharge capacity,initial reversible efficiency and rate characteristics.

In addition, the lithium secondary battery manufactured by theabove-described method was charged/discharged for 50 cycles at 25° C. ina driving voltage range from 3.0V to 4.4V under the condition of 1 C/1C, and then a ratio of the discharge capacity at the 50^(th) cyclerelative to the initial capacity (capacity retention) was measured.

Among the results of the measured battery capacity and lifetimecharacteristics, the measurement results for the positive electrodeactive material having a precursor composition ofNi_(0.8)Co_(0.1)Mn_(0.1) are shown in Tables 3 and 4 below, and themeasurement results for the positive electrode active material having aprecursor composition of Ni_(0.94)Co_(0.03)Mn_(0.03) positive electrodeactive material are shown in Tables 5 and 6 below.

TABLE 3 Initial charge Initial discharge Initial reversible capacitycapacity efficiency Classification (mAh/g) (mAh/g) (%) Example 1 224.9213.6 95.0 Example 2 225.0 213.3 94.8 Comparative 225.1 210.4 93.5Example 1 Comparative 225.2 211.5 93.9 Example 2 Comparative 225.3 211.193.7 Example 5 Comparative 225.2 210.6 93.5 Example 6

TABLE 4 Lifetime character- Rate characteristics (%) istics (%)Classification 0.2 C 0.5 C 1.0 C 1.5 C 2.0 5.0 C 50 cy Example 1 97.293.1 89.6 87.7 86.0 81.4 97.3 Example 2 97.3 93.2 89.8 88.0 86.3 81.297.2 Comparative 97.4 93.4 90.1 88.1 86.6 80.6 97.3 Example 1Comparative 97.7 93.8 90.2 88.3 86.7 80.9 92.0 Example 2 Comparative97.6 93.8 90.1 88.9 86.5 80.8 93.3 Example 5 Comparative 97.5 93.6 90.188.0 86.4 80.3 94.7 Example 6

Referring to the results shown in Table 3, it can be confirmed that thepositive electrode active materials according to Examples 1 and 2 inwhich a gradient range of the average aspect ratio of the primaryparticles present in the core of the secondary particle and the averageaspect ratio of the primary particles present in the surface region ofthe secondary particle is adjusted exhibit excellent initial reversibleefficiency, compared to the positive electrode active materialsaccording to Comparative Example 1, Comparative Example 2, ComparativeExample 5 and Comparative Example 6.

In addition, referring to the results of Table 4, it can be confirmedthat, unlike the positive electrode active materials according toComparative Example 2, Comparative Example 5 and Comparative Example 6using an Nb-containing raw material, the positive electrode activematerials according to Examples 1 and 2 exhibit a similar level oflifetime characteristics to that of the positive electrode activematerial according to Comparative Example 1 not containing anNb-containing raw material in the process of preparing a positiveelectrode active material. That is, although being doped with a metalelement such as niobium (Nb) in the positive electrode active material,the positive electrode active materials according to the examples of thepresent invention may maintain lifetime characteristics and improvecapacity characteristics by adjusting a gradient of the average aspectratio of primary particles in the secondary particle.

TABLE 5 Initial charge Initial discharge Initial reversible capacitycapacity efficiency Classification (mAh/g) (mAh/g) (%) Example 3 242.8226.5 93.3 Comparative 243.7 222.3 91.2 Example 3 Comparative 243.3221.4 91.0 Example 4

TABLE 6 Lifetime character- Rate characteristics (%) istics (%)Classification 0.2 C 0.5 C 1.0 C 1.5 C 2.0 5.0 C 50 cy Example 3 96.691.4 88.5 87.2 86.2 81.9 94.1 Comparative 96.6 91.5 88.8 87.4 86.3 80.189.3 Example 3 Comparative 96.8 91.3 88.5 87.1 86.0 80.4 90.6 Example 4

Overall, it can be confirmed that, since the positive electrode activematerial according to Example 3 uses a precursor with anNi_(0.94)Co_(0.03)Mn_(0.03) composition, the initial charging capacityof the positive electrode active material according to Example 3 ishigh. However, the positive electrode active material according toExample 3 has a low initial reversible capacity due to a higher initialirreversible capacity than that of the positive electrode activematerial of Example 1.

Meanwhile, referring to the results of Tables 5 and 6, it can beconfirmed that the positive electrode active material according toExample 3 has excellent initial reversible capacity and lifetimecharacteristics, compared to the positive electrode active materialsaccording to Comparative Examples 3 and 4 using a precursor with thesame composition. Like the results shown in Tables 3 and 4, theseresults reveal that the positive electrode active materials according tothe examples of the present invention maintain the lifetimecharacteristics and improve capacity characteristics by adjusting agradient of the average aspect ratio of primary particles in thesecondary particle although the positive electrode active material isdoped with a metal element such as niobium (Nb).

Positive electrode active materials according to various embodiments ofthe present invention include primary particles enabling lithiumintercalation/deintercalation and secondary particles in which theprimary particles are aggregated, wherein the primary particles includedin the secondary particle can improve the density of the primaryparticles in the secondary particle and electrochemical characteristicsof the positive electrode active materials by adjusting a range of anaspect ratio gradient or sphericity gradient of the primary particlesincluded in the secondary particle.

Particularly, when a metal element such as niobium (Nb) is doped in thepositive electrode active material, lifetime characteristics areprobably reduced, but when a gradient range of the average aspect ratioof the primary particles present in the secondary particle and theaverage aspect ratio of the primary particles present in the surfaceregion of the secondary particle is adjusted, a decrease in lifetimecharacteristics of the positive electrode active material can beminimized, and other electrochemical characteristics (e.g., chargecapacity, efficiency, output, etc.) can also be improved.

In addition, the primary particles included in the positive electrodeactive materials according to various embodiments of the presentinvention can exhibit an aspect ratio gradient increasing from the coreto the surface region of the secondary particle, but the size of theaspect ratio gradient can be determined within a range in which thesphericity of the primary particles present in the surface region of thesecondary particle is not greatly harmed. Therefore, the primaryparticles present in the surface region of the secondary particle have ashape close to the shape of the primary particles present in the core ofthe secondary particle, rather than a rod shape.

Accordingly, the density of the primary particles in the positiveelectrode active material can be improved, and a partial strain causedby asymmetric volume expansion of the positive electrode active materialcan be effectively reduced during the charging/discharging of a lithiumsecondary battery using the positive electrode active material. Such adecrease in strain can reduce cracking after the lifetime of thepositive electrode active material. As a result, the lifetime andstability of a lithium secondary battery can be improved.

Above, while the embodiments of the present invention have beendescribed, it will be understood by those of ordinary skill in the artthat the present invention may be changed and modified in various waysby addition, alternation, or deletion of components without departingfrom the spirit of the present invention defined in the appended claims.

What is claimed is:
 1. A positive electrode active material, comprising:primary particles enabling lithium intercalation/deintercalation andsecondary particles in which the primary particles are aggregated,wherein the average aspect ratio of the primary particles has a gradientincreasing from the core of the secondary particle to the surface regionof the secondary particle, and as the content of a metal dopant in theprimary particle increases, a difference between the average aspectratio of all primary particles in the surface region of the secondaryparticle and the average aspect ratio of all primary particles in thecore of the secondary particle is reduced.
 2. The positive electrodeactive material of claim 1, wherein when a distance from the core to thesurface region of the secondary particle is R, a region where thedistance from the core of the secondary particle is 4/5R to R is a firstregion (R₁), and a region where the distance from the core of thesecondary particle is 0 to 2/5R is a second region (R₂), a ratio (W₁/W₂)of the average aspect ratio (W₁) of all primary particles in the firstregion and the average aspect ratio (W₂) of all primary particles in thesecond region is less than 2.217.
 3. The positive electrode activematerial of claim 2, wherein the ratio (W₁/W₂) of the average aspectratio (W₁) and the average aspect ratio (W₂) is more than 1.280.
 4. Thepositive electrode active material of claim 2, wherein the proportion ofprimary particles having a smaller aspect ratio than the average aspectratio (W₁) among the primary particles in the first region is 45% ormore.
 5. The positive electrode active material of claim 4, wherein aratio (W₃/W₂) of the average aspect ratio (W₃) of primary particleshaving a smaller aspect ratio than the average aspect ratio (W₁) amongthe primary particles in the first region and the average aspect ratio(W₂) is more than 0.984 and less than 1.465.
 6. The positive electrodeactive material of claim 4, wherein a ratio (W₄/W₂) of the averageaspect ratio (W₄) of the primary particles having a higher aspect ratiothan the average aspect ratio (W₁) among the primary particles in thefirst region and the average aspect ratio (W₂) is more than 1.793 andless than 3.076.
 7. The positive electrode active material of claim 2,wherein the average aspect ratio (W₁) is less than 4.083.
 8. Thepositive electrode active material of claim 2, wherein the averageaspect ratio (W₂) is less than 1.842.
 9. The positive electrode activematerial of claim 1, wherein the metal dopant comprises niobium (Nb).10. The positive electrode active material of claim 9, wherein theprimary particle is represented by Formula 1 below:Li_(w)Ni_(1(x+y+z+z′))Co_(x)M1_(y)M2_(z)Nb_(z′)O₂  [Formula 1] (Here, M1is at least one selected from Mn and Al, M2 is at least one selectedfrom P, Sr, Ba, B, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe,Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd and Cu, M1 and M2 are differentelements, and 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, and 0≤z′≤0.20.)11. The positive electrode active material of claim 10, wherein thehigher the z′ value, the lower the ratio (W₁/W₂) of the average aspectratio (W₁) and the average aspect ratio (W₂).
 12. The positive electrodeactive material of claim 1, further comprising: a coating layer coveringat least a part of the interface between the primary particles and thesurface of the secondary particle, wherein the coating layer comprisesat least one oxide represented by Formula 2 below:Li_(a)M3_(b)O_(c)  [Formula 2] (Here, M3 is at least one selected fromNi, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au,B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd and Nd, and 0≤a≤10,0≤b≤8, and 2≤c≤13.)
 13. A positive electrode comprising the positiveelectrode active material according to claim
 1. 14. A lithium secondarybattery using the positive electrode of claim 13.